Turbulent structures and budgets behind permeable ribs

Abstract

Different rib geometries are traditionally used to improve the heat transfer and mixing enhancement in different industrial applications i.e. heat exchangers, cooling passages of gas turbine blades and fuel elements of nuclear reactors etc. These types of flows are complex due to the flow separation, recirculation bubble, three dimensionality and unsteadiness. Additional secondary vortical structures are present behind the permeable ribs due to the flow through the perforations. The use of particle image velocimetry (PIV) for these flows is expected to improve the understanding of underlying flow structures. However, the use of PIV for these flows in literature has been limited to mean velocities and their second moment statistics i.e. rms quantities. The present study presents the detailed turbulent statistics i.e. mean and rms velocity, higher order moments, the quadrant decomposition of turbulent shear stress producing motions, large and small scale contributions from wavelet analysis and components of the turbulent kinetic energy budgets i.e. convection, diffusion, dissipation and production etc. from two-component and stereo PIV measurements. The availability of instantaneous velocity field with high spatial resolution in PIV measurements offers new possibilities in calculation of differential quantities required for detailed kinetic energy budget calculation. The flow past different surface mounted permeable rib geometries i.e. solid, slit, split-slit and inclined split-slit ribs have been investigated. The assumptions traditionally used in literature for the calculation of budget terms have been verified by the PIV measurements. The coherent structure statistics calculated using the velocity gradient tensor invariant values, wavelet and quadrant analysis are successful in presentation of large scale and small scale turbulent contributions. Overall, this study demonstrates the effectiveness of PIV technique for the detailed turbulent structures investigation of complex flows.